For every natural phenomenon there are different levels of
explanation. I like to call these "why" values. For the first value of
why, the question "Why is the sky blue?" can be answered with some
simple hand-waving about light from the sun passing through our
planet's atmosphere. For a value of why + 2 (also known to parents as,
"But whyyyyy?") you have to start talking about the fact that light
comes in different colors.